After doing many trades, and being in Canada so 90% of my trades are international I have some observations.
karsal's info for us going south is solid. Another factor is how friendly you are with your PO. Documents and paper products cost less than other objects. So if you are sending from here something that weighs the same as a bubble filled with paper it is still more expensive than the bubble full of documents. My statement once was that cards are made of cardboard, which is paper, so my PO workers who I have got to know well always charge me the document rate on bubbles.
Also, yes if your PO worker in the States is a stick in the mud bubbles coming here on small to medium size trades are more expensive than us going there, however it depends alot on your relationship with your PO. I have had bubbles from the States arrive with under $4 postage, and similar packages arrive with $10 postage. Once the packages get larger though you seem to come out ahead, and As stated for us to get a tracking number can cost an extra $14-20 bucks!
Edit: I have no idea why the font changed